Now Israel loved Joseph best of all his sons, for he was the child of his old age; and he had made him an ornamented tunic.
Yaakov made Yosef a “ketonet passim/colorful tunic”. The word “pas/palm” refers to the fact that it’s sleeves extended to Yosef’s palms. Others say, that pas refers to the fine fabric that the tunic was made from. It could be folded in such a way that it would fit into the palm of one’s hand.
Some say that this was Adam’s tunic that had been passed down from generation to generation and could only be worn by a firstborn son. It was eventually given to Reuven but Reuven forfeited his right to wear it due to his sin. As such, it was given to Yossef.
Esav gave Yaakov the tunic when he sold him his birthright because only a first born could wear it.
The tunic weighted 2 selaim, about 3 ounces.
The sages learn from there that a father shouldn't show favoritism to one child over the others.